It can't be cast on Emrakul, as it is a colored spell.This card, similar to creature enchantments is begging your opponent to 2-for-1 you by killing the creature you target. Fling only has that problem if the spell if countered. The question you have to ask yourself with this card is "what are the odds that I will have a creature with at least 5 power when I want to cast this?"The person suggesting mossbridge troll has the right idea since it regenerates for free. Plated Slagwurm is another good target.

I'm very reluctant to replace fling with this. The extra colorless may not seem like much, but an unexpected turn four win from a soulstoked Nova Chaser is worth losing the creature, especially if you champion Flamekin Harbinger. Also, if you're looking to deal double damage with an unblocked attacker at instant speed, Double Cleave is almost always better.